1. Field of the Invention
Embodiments of the invention generally relate to milling within a wellbore. More particularly, the invention relates to straightening a shifted or restricted wellbore by reciprocating a flexible broach axially within the wellbore.
2. Description of the Related Art
Hydrocarbon wells typically begin by drilling a borehole from the earth's surface to a selected depth in order to intersect a formation. Steel casing lines the borehole formed in the earth during the drilling process. This creates an annular area between the casing and the borehole that is filled with cement to further support and form the wellbore. Thereafter, the borehole is drilled to a greater depth using a smaller diameter drill than the diameter of the surface casing. A liner may be suspended adjacent the lower end of the previously suspended and cemented casing. In general, the diameter, location, and function of the tubular that is placed in the wellbore determines whether it is known as casing, liner, or tubing. However, the general term tubular or tubing encompasses all of the applications.
Shifting of the wellbore caused by pressure changes in the wellbore, swelling of surrounding formations, subsidence, earth movements, and formation changes can deform, bend, partially collapse, or pinch downhole tubulars. Therefore, a cross section of downhole tubulars becomes more irregular and non-round over time. Further, the path through the wellbore may become crooked, offset, or bent at an abrupt angle due to the shifting. Bends in the wellbore and deformed tubulars that define the bore can obstruct passage through the bore of tubing, equipment, and tools used in various exploration and production operations. For example, the bend may prevent a sucker rod from functioning and cause production to cease. Even if the tool can pass through the bore, these obstructions often cause wear and damage to the tubing, equipment, and tools that pass through the obstructed bore.
Current remediation operations to correct bends in the wellbore utilize rotational mills. The rotational mills have cutting surfaces thereon that rotate along the shifted section of the wellbore to remove casing and surrounding materials, thereby reducing the severity or abruptness of the angle. The mill provides a straighter path through the wellbore and reestablishes a bore that a round tubular can pass through. A liner secures in place across the milled portion in order to complete the remediation operation.
However, there exist several problems with using rotational mills for shifted wellbore remediation. In operation, one end of a rigid mill contacts an opposite side of the casing at the shift in the wellbore and places large side loads on the mill along the area being milled. The side loads cause rigid mills to fail prematurely resulting in the expense of replacement and repeated trips downhole to complete the milling process. Further, the mill can sidetrack away from the wellbore if the mill is not kept within the portions of the wellbore on either side of the shifted area during the milling procedure. Recently, rotating mills disposed on flexible members such as cable have been used to initiate the milling process at the shifted portion of the wellbore, thereby permitting a second mill that is run in separately to complete the milling process. Milling by rotation of a flexible mill is described in detail in U.S. Pat. No. 6,155,349, which is hereby incorporated by reference in its entirety. Requiring two trips downhole to complete the milling of the shifted section of the wellbore requires additional time at an added expense. Further, the flexible member may prematurely fatigue due to the stresses caused by the rotation during the milling.
Mills are used in various other wellbore remediation and completion operations. Generally, mills may remove ledges and debris left on the inside diameter of the tubulars such as excess cement, equipment remnants, burrs on the tubular itself, or metal burrs on the inside of the casing around a milled window. Well tubulars may become plugged or coated during production from corrosion products, sediments, hydrocarbon deposits such as paraffin, and scum such as silicates, sulphates, sulphides, carbonates, calcium, and organic growth. Thus, milling operations can remove the debris that collects on the inside surface of the tubular in order to prevent obstruction of the passage of equipment and tools through the bore of the tubulars. Further, mills can be used to elongate windows and straighten the angle into a lateral wellbore.
Therefore, there exists a need for an improved tool and method of milling within a wellbore that reduces stress and fatigue from rotation. There exists a further need for an improved method for remediation of a shifted section of wellbore with a single trip downhole.